1. Field of the Invention
The present invention relates to a band pass filter, and more particularly to a delay time compensation band pass filter in which the deviation of the group delay time in the pass band is small.
2. Background Art
A communication apparatus which communicates information by radio or with wire is configured by various high-frequency components such as amplifiers, mixers, and filters. Among such components, a band pass filter is formed by arranging a plurality of resonators to exert a function of allowing only a signal of a specific frequency band to pass through the filter.
In a communication system, a band pass filter is requested to have a skirt characteristic which does not cause interference between adjacent frequency bands. A skirt characteristic means the degree of attenuation in a range from an end of the pass band to the stop band. When a band pass filter having a steep skirt characteristic is used, therefore, it is possible to effectively use the frequency.
On the other hand, a band pass filter in a communication system is requested to have a group delay characteristic which is flat in the pass band. Usually, group delay compensation is performed by means of a real zero and a complex zero of a transfer function related to a complex frequency s.
In order to flatten a group delay characteristic, a method in which an equalizer is connected to a subsequent stage of a filter is sometimes employed. However, this method has a problem in that the insertion loss is increased by the loss of the equalizer.
As a filter in which a filter circuit itself performs group delay compensation without using an equalizer, a canonical filter is reported in IEEE Transactions on Microwave Theory and Techniques, Vol. 18 (1970), p. 290. In the filter, first to N-th resonators are sequentially main-coupled, and the first and N-th resonators, the second and (N−1)-th resonators, and the like are sub-coupled, so that an (N/2−1) number of sub-couplings exist in total.
In a canonical filter of six or more stages, flexible group delay compensation is enabled by providing real and complex zeros. Conventionally, this has been applied to a waveguide filer or a dielectric filter. In a canonical filter, however, a zero of a transfer function depends on complicated interactions of all sub-couplings, thereby causing a problem in that it is difficult to adjust the filter characteristic. When a large number of resonators are arranged in the form of a canonical filter with using a planer circuit such as a microstrip line, a strip line, or a coplanar line, it is very difficult to suppress unwanted parasitic couplings, thereby producing a problem in that a desired characteristic is hardly obtained.
As a modification of a canonical filter, a waveguide filer is reported in IEEE Transactions on Microwave Theory and Techniques, Vol. 30 (1982), p. 1300. In this filter, however, resonators are coupled in a more complicated manner than a usual canonical filter, and hence it is difficult to adjust the filter characteristic. There is a problem in that it is very difficult to realize such a filter with using a planar circuit such as a microstrip line, a strip line, or a coplanar line.
As a filter in which a steep skirt characteristic and a flattened group delay characteristic are simultaneously realized with using a planar circuit, known is a cascaded quadruplet filter reported in IEEE Transactions on Microwave Theory and Techniques, Vol. 43 (1995), p. 2940. The cascaded quadruplet filter has a configuration in which four resonators are formed into a set to form one sub-coupling. A steep skirt characteristic can be realized by disposing an attenuation pole due to a pure imaginary zero of a transfer function, and group delay compensation can be realized by a real zero. Since zeros of a transfer function correspond to sub-couplings in a one-to-one relationship, the filter has an advantage that a configuration is enabled in which the filter characteristic is easily adjusted and unwanted parasitic couplings are suppressed in a planar circuit. In such a cascaded quadruplet filter, however, it is impossible to realize a complex zero of a transfer function, and hence there is a problem in that flexible group delay compensation cannot be performed.
An example of a cascaded quadruplet filter is an 8-stage waveguide filter reported in IEEE Transactions on Microwave Theory and Techniques, Vol. 29 (1981), p. 51. This filter is designed by rotation-transforming a coupling coefficient matrix of a circuit in which the coupling between first and eighth stages of an 8-stage canonical filter is made zero. Delay compensation is performed by disposing one real zero. Since a complex zero is not provided, however, the delay compensation cannot be sufficiently performed.
A method of realizing a filter circuit in which a steep skirt characteristic is realized by disposing an attenuation pole due to a pure imaginary zero of a transfer function, and group delay compensation is performed by a real zero is described also in JP-A-2001-60803. In the method, however, it is impossible to use a complex zero of a transfer function, and hence there is a problem in that flexible group delay compensation cannot be performed.